1. Field of the Invention
The present invention relates to a method for decomposing nitrogen fluoride or sulfur fluoride and a decomposing reagent used therefor.
In the present specification, the nitrogen fluoride designates a compound which comprises fluorine and nitrogen as essential constituent elements and which easily volatilizes. A typical example of nitrogen fluoride is nitrogen trifluoride (NF3). Sulfur fluoride designates a compound which comprises fluorine and sulfur as essential constituent elements and which easily volatilizes. A typical example of sulfur fluoride is sulfur hexafluoride (SF6). xe2x80x9cA compound which easily volatilizesxe2x80x9d herein denotes a compound which is a gas at room temperature and normal pressure, or a compound which is a liquid at room temperature but which forms a gas mixture containing at least 0.01% by volume of the compound as a vapor it an inert gas is co-present.
2. Description of the Related Art
Since nitrogen fluoride and sulfur fluoride defined above are thermally stable, they are used, for example, as gases for etching or cleaning in processes for manufacturing semiconductor devices. However, nitrogen fluoride and sulfur fluoride are substances which are suspected to have an influence on global warming because they have a large global warming potential (GWP) value and remain in the atmosphere without decomposition when released in the air, and it is said that they are preferably decomposed after use. Accordingly, decomposition of used nitrogen fluoride and sulfur fluoride into nontoxic substances is required.
A combustion decomposition method, a reagent decomposition method, a catalytic decomposition method, and the like have heretofore been proposed as technologies for decomposing nitrogen fluoride.
Decomposition of nitrogen fluoride by the combustion method unavoidably forms by-product NOx. The combustion method, therefore, requires removal of the by-product and an additional treatment of the fluorine component. Accordingly, the combustion method is not efficient from the standpoint of recovering the fluorine component. The proposed reagent decomposition methods and catalytic decomposition methods require special treating conditions to increase the decomposition efficiency, and special treatment is required for recovery of the fluorine component formed by the decomposition. Accordingly, the decomposition operation in situ where nitrogen fluoride is used (generation source of a nitrogen fluoride gas) cannot be simply conducted.
Furthermore, no method for efficiently and completely decomposing sulfur fluoride such as sulfur hexafluoride (SF6) into nontoxic substances has ever been known.
An object of the present invention is, therefore, to provide a method for decomposing nitrogen fluoride or sulfur fluoride, which allows decomposition of nitrogen fluoride or sulfur fluoride at a high efficiency by a simple operation and efficient recovery of decomposed fluorine, and to provide a decomposing reagent therefor.
The above object is solved in accordance with the present invention by providing a method for decomposing nitrogen fluoride or sulfur fluoride comprising contacting at least one of nitrogen fluoride and sulfur fluoride gases with a solid reagent containing elemental carbon and one or more of the alkaline earth metal elements.
More specifically, nitrogen fluoride can be decomposed by contacting a nitrogen fluoride gas with the reagent as mentioned above at a temperature of 200xc2x0 C. or more. In the decomposition, formation of by-product carbon fluoride can be inhibited by contacting the nitrogen fluoride gas with the reagent at a temperature of 200 to 450xc2x0 C. or at a temperature of not less than 700xc2x0 C. Moreover, formation of by-product nitrogen oxide can be inhibited by contacting the nitrogen fluoride gas with the reagent at a temperature of 350xc2x0 C. or more.
Furthermore, when the reagent mentioned above further contains an alkali metal element, a nitrogen fluoride gas can be similarly decomposed at a contact temperature of 200xc2x0 C. or more. Moreover, formation of by-product carbon fluoride can be inhibited by contacting the nitrogen fluoride gas with the reagent at a temperature of 200 to 450xc2x0 C. or at a temperature of not less than 700xc2x0 C. Furthermore, formation of by-product nitrogen oxide can be inhibited at the contact temperature of 250xc2x0 C. or more, lower than the lower limit of the contact temperature in the above case where the reagent does not contain an alkali metal element.
Similarly, sulfur fluoride can be decomposed by contacting a sulfur fluoride gas with a solid reagent containing elemental carbon and an alkaline earth metal element at a contact temperature of 300xc2x0 C. or more. In the decomposition, formation of by-product sulfur oxide such as SO2 can be inhibited by contacting the sulfur fluoride gas with the reagent at a temperature of 450xc2x0 C. or more.
Furthermore, a sulfur fluoride gas can be similarly decomposed at a contact temperature of 300xc2x0 C. or more when the reagent further contains an alkali metal element. In addition, in this case, formation of by-product sulfur oxide can be inhibited at a temperature lower than in the case where the reagent does not contain an alkali metal element. More concretely, formation of by-product SO2 can be inhibited from a temperature of 350xc2x0 C. or more.